1. Field of Invention
The present invention relates generally to the field of electronic transmission line devices and, more particularly, to directional couplers.
2. Discussion of Related Art
Directional couplers are passive devices used in many radio frequency (RF) applications, including for example, power amplifier modules. Directional couplers couple part of the transmission power in a transmission line by a known amount out through another port, in the case of microstrip or stripline couplers by using two transmission lines set close enough together such that energy passing through one is coupled to the other. As illustrated in FIG. 1, a directional coupler 100 has four ports, namely an input port P1, a transmitted port P2, a coupled port P3, and an isolated port P4. The term “main line” refers to the transmission line section 110 of the coupler between ports P1 and P2. The term “coupled line” refers to the transmission line section 120 that runs parallel to the main line 110 and between the coupled port P3 and the isolated port P4. Often the isolated port P4 is terminated with an internal or external matched load, for example, a 50 Ohm or 75 Ohm load. It is to be appreciated that since the directional coupler is a linear device, the notations on FIG. 1 are arbitrary. Any port can be the input port, which will result in the directly connected port being the transmitted port, the adjacent port being the coupled port, and the diagonal port being the isolated port (for stripline and microstrip couplers).
Microstrip and stripline couplers are widely implemented in power amplifier modules, particularly those used in telecommunications applications, using multi-layer laminate printed circuit boards (PCBs) due to ease of fabrication and low cost. Conventionally, these couplers are realized by placing the main RF line 210 and the coupled line 220 on two vertically adjacent PCB layers and maintaining overlap of the two structures to provide the RF coupling, as shown in FIG. 2.